Gradient-enhanced multiple quantum NMR spectroscopy has been used to detect 13C-label in primary metabolites isolated from plants under different physiological conditions. The distribution of label reflects the relative activities of competing metabolic pathways, such as anaplerotic and catabolic metabolism of glycolytic intermediates. We have focused on 13C-13C pairs, detected via selective magnetization in protons attached to one of the 13C atoms in the pair. This approach offers the high sensitivity of indirect 1H detection; the selectivity necessary to observe 13C label in complex mixtures; and the capacity to detect 13C in groups lacking directly attached 1H, such as carboxylates. The application of these methods for quantitative analysis has been evaluated.